1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display and a driving method thereof that is adapted to improve display quality by preventing flickers and DC image sticking.
2. Discussion of the Related Art
A liquid crystal display controls the light transmittance of liquid crystal cells in accordance with video signals, thereby displaying a picture. An active matrix type liquid crystal display actively controls data by switching data voltages supplied to liquid crystal cells using thin film transistors (“TFTs”) that are formed at each liquid crystal cell Clc, as shown in FIG. 1, thereby increasing the display quality of motion pictures. As shown in FIG. 1, reference label “Cst” represents a storage capacitor for keeping data voltages charged in the liquid crystal cell “Clc,” “DL” represents a data line to which the data voltages are supplied, and “GL” represents a gate line to which scan voltages are supplied.
The liquid crystal display is driven by an inversion method where polarities are inverted between adjacent liquid crystal cells and by a unit of a frame period in order to reduce the deterioration of liquid crystals and to decrease DC offset components. If any one polarity out of two polarities of the data voltage is dominantly supplied for a long time, a residual image is generated. Such a residual image is called “DC image sticking” because the residual image is generated by a voltage of the same polarity repeatedly charged in the liquid crystal cell. One such example occurs when data voltages of an interlace method are supplied to the liquid crystal display. In the interlace method, data voltages to be displayed on the liquid crystal cells (hereinafter, referred to as “interlace data”) exist only in odd-numbered horizontal lines during odd-numbered frame periods and only in even-numbered horizontal line in even-numbered frame periods.
FIG. 2 illustrates a waveform diagram representing an example of a data voltage of an interlace method supplied to a liquid crystal cell Clc. For purposes of example, the data voltage of FIG. 2 is supplied is any one of the liquid crystal cells disposed on an odd-numbered horizontal line. As shown in FIG. 2, the liquid crystal cell Clc is supplied only with positive voltages for an odd-numbered frame period and only with negative voltages for an even-numbered frame period. In the interlace method, high positive data voltages are supplied only for the odd-numbered frame periods to the liquid crystal cells Clc disposed on the odd-numbered horizontal lines. Thus the positive data voltage, like the waveform shown within the box in FIG. 2, becomes more dominant than the negative data voltage over four frame periods, thereby causing DC image sticking to occur.
FIG. 3 illustrates an image showing an experimental result of DC image sticking generated due to interlace data. If an original image, like the image shown on the left in FIG. 3, is supplied to a liquid crystal display panel using the interlace method for a fixed time, the data voltage of which the polarity is changed by the unit of a frame period has its amplitude changed in the odd-numbered frame and in the even-numbered frame. As a result, if a data voltage of an intermediate gray level, e.g., the gray level of 127, is supplied to all of the liquid crystal cells Clc of the liquid crystal display panel after the original image (i.e., the left image), DC image sticking occurs showing a dim pattern of the original image, like the image shown on the right in FIG. 3.
As another example of DC image sticking, if an unchanging picture is moved or scrolled at a fixed speed, DC image sticking may be generated because the voltage of the same polarity is repeatedly accumulated in the liquid crystal cell Clc depending on the scroll speed (or moving speed) and the size of the scrolling picture (i.e., moving picture). Such an example is shown in FIG. 4. FIG. 4 illustrates an image showing an experimental result of DC image sticking that occurs when moving an oblique line or character pattern at a fixed speed.
In the liquid crystal display, the display quality of motion pictures is not only reduced by DC image sticking, but also by a flicker phenomenon generated by a brightness difference that is visually perceived. Accordingly, in order to increase the display quality of the liquid crystal display, the flicker phenomenon the DC image sticking need to be prevented.